Agree. If plug wires are incorrect, it won't even idle properly.
If stutters above idle, try:
- remove and clean the carbs. If engine sits more than 6 months (longer the riskier), its old gas remaining in its carb and VRO system will sludge (varnish) up and plug its many small holes. May need to replace the needle and seat within the carbs as well. Especially the high speed jet needle holes.
- Replace inline gas filters as well.
- Replace with new gas. Use high-octane / non-ethanol fuel - if possible.
- Get its VRO pump flow inspected as well.
- If not done so already, install Water/Fuel separator system as well.
Note: Do a compression test of each cylinder. Do provide us the numbers. A compression test kit can be bought at many automotive stores.
Also... If you don't trust the oil side of its VRO system, you can easily convert to pre-mix. Some folks love premix and some don't. My Johnson 115 was converted to pre-mix (50:1) and its been working great ever since. If unkown maintenance history, do change its bottom end gear oil as well - even if its oil looks brand now.
If shutter / entire engine vibration above 2K, do inspect its prop as well. A bent or large nick in one blade can vibrate the engine as well. re: The tail wagging the dog body thing.
Hope these ideas help.
